This invention relates to an integrally-bonded composite of a substrate part or layer or metal or the like and a thermoset plastic layer molded in place under heat and pressure against the substrate layer and integrally and chemically bonded to it by an intervening elastomeric bonding and shock-resisting interface layer; and to a method of making such composite.
Numerous situations exist, especially in the electrical field, where it would be highly advantageous to have a metal part or layer bonded to a molded thermoset plastic wall or layer with a bond which has a very high resistance to separation and which forms and imprevious seal between the two parts. Thermosetting plastic molding compositions, such as glass-reinforced polyester compositions, find wide usefulness in the electrical field, as for forming supports and housings for electrical apparatus. Such resins would have even greater usefulness if they could be made to bond to metal inserts and walls with a bond which would provide reliable high resistance to separation and leakage. In many applications it is essential that the molding and its interface with the metal be free of voids or separations spaces, in order to prevent corona discharge in such spaces which would degrade the plastic, to prevent the collection of moisture which would cause corrosion of the metal, and/or to prevent leakage along the interface between the two layers. Also, in many instances, the adjoining metal and plastic parts must provide such characteristics under adverse conditions such as large temperature changes and differences and thermal shock. These properties may be obtained or enhanced by the presence of an interface layer which provides some degree of elasticity at the interface, and the interface layer may provide other properties, for example, by containing carbon or other components giving it electrical conductivity.
U.S. Pat. No. 3,388,212 to Nichols et al. shows a plastic bushing cast about an electrical conductor and in an enclosure wall opening. A coating of elastomer is vulcanized to the wall edge and the insert before the bushing is formed and is said to form a seal with the bushing. However, there the seal is between a completely vulcanized rubber layer and a cast epoxy plastic material which has adhesive properties, and that process is not applicable to other resin materials such as polyester resin materials which do not possess similar adhesive properties but which otherwise have highly desirable properties. Also, the seal and bonding there obtained is not as good as that obtained by the present invention.
In the copending application of myself and others, Ser. No. 292,187, filed Sept. 25, 1972, now U.S. Pat. No. 3,801,727, it is proposed to provide an electrical transformer to be buried under ground. This requires a sealed housing which will withstand the various adverse physical and chemical conditions of a direct-buried environment, and which provides under such adverse conditions a substantially complete barrier to leakage or diffusion of moisture through the housing walls to contaminate the transformer oil or cause deterioration of the electrical insulation, for example, to limit entry to not more than 50 parts moisture per million parts of transformer oil over a life of 20 to 30 years.
Molded plastic materials, such as molded glass-reinforced polyester resins, have a number of advantageous properties for this application. However, walls molded of these resins are not completely impervious and moisture passes through them by diffusion, and it has been found necessary to provide such transformer housings with a metal liner to form a more effective vapor barrier. It is essential in the lined plastic housing that the metal lining and the plastic be securely and permanently bonded so that there can be no pockets at the interface in which corona could occur to cause degradation of the plastic or in which moisture could collect to cause corrosion of the metal, or where gas could expand under temperature changes to separate the liner layer from the plastic layer of the wall to create larger pockets.
Also, in such an underground transformer housing it is necessary to provide high voltage and low voltage conductors through the housing wall, as through its cover, and it is especially desirable that such terminals be provided as inserts in the molded housing as it is molded.
The present invention provides a method of forming a composite wall for such an underground transformer in which a metal vapor barrier liner is integrally bonded to a surrounding plastic wall molded in place against the liner. It also provides a method of forming a composite between the molded-in conductor terminal inserts and the molded resin wall, in which the metal inserts are securely bonded to the wall and sealed thereto with a seal which is highly impervious to leakage. The method is also applicable to produce other composites, such as terminal insulating bushings with metal conductors molded therein for other applications, circuit boards having a conductive layer bonded to a supporting molded plastic wall, chemical plumbing and pump parts in which a thin non-corrosive metal liner is combined with an inexpensive mechanical support molded of thermoset molding composition, etc. With the improved method of forming a securely bonded composite, various other applications will be evident to those skilled in the art.